A type of optical pick-up devices currently provided employs semiconductor laser to read information recorded on optical disks, such as compact disk (CD) and compact disk-read only memory (CD-ROM). This optical pick-up device irradiates a train of pits on a signal recording surface with laser beam while applying focusing servo and tracking servo controls to an objective lens for pick-up to reproduce audio, video and data signals and the like.
In recent years, optical disks of higher density have been developed to record moving pictures for a long period of time. For example, a digital video disk (DVD) standard has been proposed for recording approximately 5 G bytes of information on one side of an optical disk the diameter of which is the same as that of CD-ROM, i.e. 12 cm.
According to this standard, the transparent substrate of the DVD has a thickness of approximately 0.6 mm.
Optical lens for pick-up is designed taking into consideration the thickness of the transparent substrate of the optical disk to be played back and the wavelength of the semiconductor laser used. Thus, an optical disk the transparent substrate of which is not as designed in thickness cannot be played back since a laser beam is not focused on the signal recording surface of the optical disk.
For example, an objective lens designed for a DVD with a transparent substrate of 0.6 mm in thickness cannot focus a laser beam on the signal recording surface of a CD with a transparent substrate of 1.2 mm in thickness so that the CD cannot be played back.
Therefore the present invention contemplates an optical pick-up device capable of employing the optical activity of liquid crystal to record on and/or playback of optical disks each having a transparent substrate of a different thickness and also capable of reducing loss of laser beam due to the liquid crystal.